The present disclosure relates to packaging structures for photovoltaic cells, and more particularly to packaging structures for photovoltaic cells employing solar concentrators and methods of manufacturing the same.
Solar concentrator systems including multiple refractive or reflective optical elements that concentrate sunlight on multiple spatially separated photovoltaic elements present challenges for electrical connection and cooling as the level of solar concentration increases.
It has been demonstrated that it is possible to operate triple junction photovoltaic concentrator cells above 2000 suns (˜200 W/cm2). Operation of concentrator photovoltaic systems at high concentrations is constrained by the ability to cool the photovoltaic elements. Operation of concentrator photovoltaic systems becomes costly and more difficult as the concentration increases, and especially above the areal power density level of 50 W/cm2. As the cell area increases, the use of an auxiliary heat exchange structure becomes necessary above and beyond a simple flat conducting surface and cost goes up. As the cell area decreases the number of connections goes up also increasing the cost.
Photovoltaic elements used for solar concentrators are typically constrained to operation at or below 100° C. for both cell efficiency and lifetime considerations. Lifetime expectations for solar systems are on the order of 20 to 30 years. Operation is desired in desert climates with high isolation where ambient temperatures exceeding 45° C. occur.
The following is a numerical example of the thermal budget for a cooling system operating at an ambient temperature of 40° C. and a chip temperature less than 85° C. In a first photovoltaic system that uses a photovoltaic element having a size of 1 cm2 and a solar concentration of 150 W/cm2, the overall cooling system performance requirement is 0.30° C./W. This combination requires a high performance cooling system with an associated high cost.
In a second photovoltaic system, a photovoltaic element having the size of 5 mm×5 mm is employed with the same level of solar concentration as above. The cooling system performance requirement is 1.2° C./W for the second photovoltaic system. Thus, the second photovoltaic system can be cooled with much less cooling cost than the first photovoltaic system.
As the chip size is reduced, the thermal spreading becomes easier. In the limit heat source approaches a point spreading in 3 dimensions. However, the number of photovoltaic elements and connections are increased by a factor of 4 whenever the lateral dimensions of a photovoltaic element are reduced by a factor of 2. Thus, the cost of providing the photovoltaic elements and connections can increase exponentially with reduction of the size of the photovoltaic elements.
Many solar concentrator systems comprise an array of concentrating Fresnel optical elements comprised of acrylic or silicone on glass material. Light from these concentrating elements is focused on cells mounted on a supporting structure and contained within an enclosure to mechanically support the individual elements and protect them from the environment. As the solar concentration is increased (the area of the photovoltaic surface decreases relative to the light collection optics area), the distance between photovoltaic elements in an array increases. To maintain the cost performance of the cooling system as concentration is increased, the size of the cell may be reduced with the consequence that the number of cell connections increases for a given total power as in the example above.
There are many factors to consider for optimization of high concentration photovoltaic systems. For example, higher concentration reduces the cost for semiconductor material but increases cooling costs. Further, larger die size increases cooling costs, but smaller die size increases the number and cost of electrical connections. Further, it is desirable that the photovoltaic devices are electrically isolated up to approximately 1,700 V for safety and up to several KV for protection from lightening strikes.